Use of substantial electrical charge applied to outer hull to repel potential boarders.

This is now a cliché; you've seen it on Star Trek, Voyage to the Bottom of the Sea, and many other places. But, this is (as far as I know) the first instance of this literary masterpiece plot device.

"M. Aronnax," quietly answered Captain Nemo, "they will not enter the hatches of the Nautilus in that way, even if they were open."

I looked at the Captain.

"You do not understand?" said he.

"Hardly."

"Well, come and you will see."

I directed my steps towards the central staircase. There Ned Land and Conseil were slyly watching some of the ship's crew, who were opening the hatches, while cries of rage and fearful vociferations resounded outside.

The port lids were pulled down outside. Twenty horrible faces appeared. But the first native who placed his hand on the stair-rail, struck from behind
by some invisible force, I know not what, fled, uttering the most fearful
cries and making the wildest contortions.

Ten of his companions followed him. They met with the same fate.

Conseil was in ecstasy. Ned Land, carried away by his violent instincts,
rushed on to the staircase. But the moment he seized the rail with
both hands, he, in his turn, was overthrown.

"I am struck by a thunderbolt," cried he, with an oath.

This explained all. It was no rail; but a metallic cable
charged with electricity from the deck communicating with
the platform. Whoever touched it felt a powerful shock--
and this shock would have been mortal if Captain Nemo had
discharged into the conductor the whole force of the current.
It might truly be said that between his assailants and himself
he had stretched a network of electricity which none could
pass with impunity.